1. Field of the Invention
This invention relates to fire dampers and more particularly to a fire damper closing mechanism that closes a fire damper at two different temperatures.
2. Description of the Related Art
A major consideration in the design of commercial and residential buildings is the spread of fire and smoke in the event that a fire breaks out within the building. The walls and ceilings within the buildings serve as the primary barrier to the spread and are most effective if they have no breaks or holes. Most buildings have heating, ventilation, and air conditioning (HVAC) systems that distribute conditioned/heated air throughout the building by air ducts. The ducts are directed to the various rooms and the air enters the rooms through a vent. However, the vents and ducts penetrate the walls/ceilings, providing a hole that reduces the ability to prevent the spread of fire and smoke. To address this problem, dampers are often provided in the ducts that allow air to pass when open, but block airflow, flames, and hot gasses when closed. At elevated temperatures (such as in the case of fire) the dampers automatically close, effectively closing the duct and vent holes and restoring the full integrity of the fire and/or smoke barrier.
In many fires, death and injury are caused by smoke, not fire. It was originally thought that closing the dampers would most effectively prevent the spread of both smoke and fire. However, the majority of smoke is spread by changes in pressure, with the smoke spreading from the area with increased pressure to an area with lower pressure. Fire increases air pressure and smoke commonly spreads to adjacent rooms and/or floors having a lower air pressure. Closing the dampers around a fire does not prevent the spread of smoke from fire pressure, and in some instances can cause an increase in fire pressure.
It was then discovered that the smoke spread could be retarded by providing air pressure opposing and surrounding the fire pressure. If the fire can be kept in a positive "pressure sandwich", smoke will spread much more slowly. One way to create this pressure sandwich is through the HVAC system, which can provide positive airflow to the fire zone. In the ducts surrounding the fire zone, the dampers in the supply air ducts are opened and the dampers in the return air ducts are closed.
However, most building code standards require dampers to automatically close at predetermined temperature, such as 74.degree. Celsius (C) (165.degree. Fahrenheit), to prevent the spread of fire. This prevents the dampers from being used to create a pressure sandwich at temperatures above the closing temperature. More recently, many building code standards allow dampers to be selectively reopened after they are initially closed so they can be used to control the spread of smoke. However, at a second higher temperature, such as 180.degree. C. (350.degree. Fahrenheit) the fire is considered out of control. The damper must again close and not be allowed to reopen.
Accordingly, there is a need for a damper that closes automatically at a predetermined lower temperature and can then be selectively reopened for smoke control. The damper must then close permanently at a predetermined higher temperature.
U.S. Pat. No. 4,463,896 to Schaus discloses a fire damper equipped with two thermally responsive electric switches. The first switch closes the damper at a predetermined temperature (74.degree. C.). Control circuitry permits an override of the first switch, allowing the damper to be reopened. The second thermally responsive switch closes the damper again at a second higher predetermined temperature (180.degree. C.). One disadvantage of this damper is that it is overly complex, relying on electrical circuitry and switches. This damper also relies on an electrical motor and circuitry that consume electricity. In larger buildings having many dampers, this energy consumption can add significant operating costs. Finally, this damper is a "power to open" product that requires electrical power to open. In some applications, it is desirable to have a power to close product where the damper stays closed with power and opens when power is lost.
Imperial Damper and Louver Company provides a dual link damper closure mechanism (Model Nos. 770 and 771), that closes a conventional damper at ambient temperatures greater than 74.degree. C. The damper can be reopened by engaging a secondary link that enables the damper to function normally until the ambient temperature exceeds 180.degree. C. At this temperature, the mechanism again closes the damper and it cannot be reopened. A primary disadvantage of this device is that it does not reliably engage the secondary heat responsive device and as a result, it will not reliably reopen the damper at temperatures exceeding 74.degree. C.